1. Field of the Invention
The present invention relates to wide-angle narrowband optical filters with wavelength pass bands on the order of 0.01×10−9 meters in the visible range, and in particular to filters with fields of view in excess of 14 degrees suitable for detecting from an aircraft optical emissions from combustion in the presence of daylight.
2. Description of the Related Art
Threats to human life and property are often harkened by combustion. For example, small fires in the wilderness can become forest fires that spread to populated areas. Anti-aircraft missiles sent aloft by burning rocket fuel when launched by terrorists or enemy combatants often target civilian and military aircraft. At night, such combustion sources are readily detected by the visible light emitted during combustion. During the daylight hours, however, the visible spectrum is deluged by daylight, including direct sunlight, sunlight scattered from the sky and clouds, or sunlight reflected from objects on the ground or in the air. The visible light from combustion is often masked by daylight.
It is common practice to use infrared (IR) detectors to detect thermal emission from heat sources at electromagnetic wavelengths long compared to those of visible light. As is well known, electromagnetic waves travel at the speed of light in a vacuum and encompass a wide spectrum of wavelengths, increasing from gamma rays to ultraviolet through the visible to the infrared and beyond to microwaves and radio waves. The optical spectrum visible to the human eye is associated with wavelengths from about 400 nanometers (nm, 1 nm=10−9 meters), appearing violet to human observers, to about 750 nm, appearing red. Heat sources from a few hundred to several thousand degrees Kelvin emit light in the IR spectrum peak intensities between about 800 nm and about 20,000 nm.
The use of IR detectors in such applications is expensive. Thus few private and commercial vehicles or aircraft are equipped with IR detectors. In addition, processing IR images to determine what sources are of interest is complex and demands great processing power to reject clutter and various signatures that are not of interest. The cost of the powerful processors adds to the cost of a system based on an IR detector. The detectors and extensive processing lead to increased cost, size and weight of systems that rely on IR detectors.
An alternative approach is to detect optical emissions from the combustion process, rather than thermal emission. The optical emissions are narrow-band, are formed by atomic and molecular optical transitions excited as part of the combustion process and can occur throughout the optical spectrum. In addition, it is possible to select a combustion emission line that falls in a solar Fraunhofer absorption line. The Fraunhofer lines are narrow minima in the spectrum of light from the sun, produced by absorption of light in the cooler regions of the sun's outer atmosphere at wavelengths corresponding to the atomic and molecular transitions of materials in these regions. The light intensity within a Fraunhofer line is often only a few percent of the intensity outside the line, further increasing the contrast between combustion emission signal and ambient daylight. For example, Fraunhofer lines associated with Potassium absorption occurs at about 766.4 and 769.9 nm, have a width of about 0.02 nm, and have a central intensity about 80% lower than outside the lines. Events of interest often include combustion of trace amounts of Potassium which emits light at 766.4 nm and 769.9 nm. Therefore the combustion signal is high at the Fraunhofer wavelengths compared to sunlight, and combustion detection is more favorable at these wavelengths, among others.
To take advantage of this combustion signal, a narrowband optical filter is needed that stops sunlight in other bands and passes light in a band about 0.01 nm wide that overlaps the Potassium emissions at about 766.4 nm or 769.9 nm, or both.
As is well known, magneto-optical filters (MOF) are capable of filtering out light except in a narrow wavelength pass band. A MOF most often uses the properties of an alkali metal vapor in a magnetic field, which includes changing the polarization of light at a characteristic optical wavelength associated with transition energy for the metal. Other materials can also be used. When combined with a pair of polarizers oriented to block out light, only the light that has had its polarization changed in the metal vapor passes through the second polarizer. Therefore, only light at the transition energy wavelength passes through the filter.
Available MOF filters fall into two classes—cold cell and hot cell. The cold cell filters produce metal vapor by heating some central part of the cell and use a buffer gas to maintain a sufficient vapor population in the central part of the cell without allowing excessive diffusion of the vapor to the end windows. The hot cell filters heat the whole cell in an oven, using a cold finger to control vapor density. The cold-cell MOFs, have a limited field-of-view, are bulky and need continuous calibration to guarantee long-term stability. While prior implementations of the hot-cell, MOF can have a moderate field-of-view, they are difficult to construct in a way that minimizes polarization-inducing stresses in the cell windows and oven enclosure windows. Furthermore, the hot cell filters require bulky ovens to maintain cell temperature and control vapor density. Thus, conventional MOF implementations have a limited usefulness for monitoring large sections of earth or sky for fires, gunfire, missiles and other important combustion events. Especially on aircraft, the total volume and weight available for a combustion monitoring system, such as a missile warning system, is limited. A large array of narrow field of view cold cell MOFs, or hot-cell MOFs with bulky ovens are simply not feasible on an aircraft. Example aircraft constraints for a combustion monitoring system is a volume no larger than about 10 centimeters (cm, 1 cm=10−2 meters) by 10 cm by 10 cm (i.e., a volume less than 1000 cubic cm) and a mass no greater than 1 kilogram (kg, 1 kg=1000 grams).
Based on the foregoing, there is a clear need for wide-field of view and short length MOFs without bulky ovens. In particular, there is a need for a low cost, small size, low weight MOF that has a field of view of more than 9 degrees about a central optical axis.
The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not to be considered prior art to the claims in this application merely due to the presence of these approaches in this background section.